A Guide To Air Dried Oak Beams
While there are many technologies available today for speeding up the process of drying sawn structural grade European oak beams, air dried oak beams are still the best in terms of quality. These beams are sawn by machine and then exposed to air and left to dry naturally.
The process of air drying may take longer than kiln drying for instance, but the natural drying process eases the tension in the wood at the ideal pace while reducing moisture. Air drying results in superior quality oak beams and top joinery and structural board manufacturers insist on it.
The quality of oak beams also depends on soil type and climate, along with management, milling, seasoning and drying. Oak timber is graded based on purpose, and within these grades there are further quality grades. For instance, dried structural grade European oak beams are graded from QBA to QB4 on the basis of frequency of splits or knots on the beams. However, irrespective of the grade of joinery oak supplied all joinery oak grades go through the same milling and drying process.
Optimal Growing and Harvesting of Oak
Oak has been around since after the last Ice Age that began around 2.6 million years ago replacing pine and birch forests that spread north as the ice receded. Today, oak is the climax community across most of Europe. This means that oak is part of the endpoint reached through the processes of ecological succession in these areas, because oak leaves have evolved remarkably to be able to cope with local soils and local climates. However, this also means that taking a seedling from one part of the continent and transplanting it in another part will take away the genetic advantage it would have had on native soil.
Top harvesters and timber merchants pay special attention to this aspect of plant genetics during oak harvest. At the time of harvest, the best couple of trees per acre are left to spill their seed. These trees create a carpet of oak seedlings on the forest floor with very small spaces between them. As a result, the seedlings are forced to compete for sunlight in order to survive. Only the dominant trees survive, and these are the ones that grow quicker and straighter than the weaker ones which die out.
Oak can grow very quickly in such conditions, typically growing about 1 meter in a year. The trees are usually ready to harvest after 70 to 120 years, leaving behind a few 'mother trees' to continue the cycle.
There is also little need for thinning and pruning as well because of the natural fight for dominance in a forest of trees. There is a great benefit for woodworkers in this. The wound created by pruned branches do heal over naturally but they tend to leave behind knots in the trees. These knots are not easy for the joiner to work with. The fewer knots that a tree has, the less trouble it creates for the joiner that has to work with the planks cut from that oak tree.
This process of growing and harvesting oak keeping genetics in mind is not the usual method prevalent in the timber industry. But it is the ideal model in sylviculture followed by superior manufacturers.
Treating the Timber
Once an oak tree has been felled, it has to be treated immediately. It should either be planked at once or treated with water sprinklers. This is to prevent the wood from drying out too fast, which can cause the timber to split. The water also helps to control the amount of damage that can be caused by boring insects at this early stage.
Logs chosen for planking are peeled and sawn into boards immediately. Peeling removes stones on the bark that can lead to irregular cuts and damaged saw blades in the mill. These planks will gradually dry and shrink to the thickness desired. Crown boards shrink less than heart boards. This means the log has to be cut at varying thickness along its length so that it shrinks to a uniform thickness as it dries. Immediately after milling, the logs are sprayed with insecticides and then graded for quality. They are then air-dried.
The Air-Drying Process
Graded and planked oak logs are first prepared for air-drying by being placed on a level base to dry with layers of sticks that create space for airflow. In the first year, 10mm sticks are used, followed by 20m in the second year. This keeps the drying slow and prevents the development of shakes. The boards that have had at least a year of drying in this manner for every inch of thickness can now be air dried (or placed in a kiln).
The time the beams take to air dry (from a few months to years) depends on the climatic conditions and the airflow around the area. The best grade of oak is created by controlling the airflow and moisture loss through the timber pile. It is possible to increase the rate of air drying by artificially creating a consistent and continuous flow of air through the stack of timber. The rate of moisture loss is controlled by coating the beams with an impermeable substance. However, the quality of the resulting air dried oak beams is reward enough despite the long process. The beams are graded again after air-drying.
Choosing the Right Oak Beams Based on Purpose
Air dried structural beams are ideal for structural uses. Since air dried oak is dried naturally, it will contain some splits, surface checking (cracks on the surfaces and ends), and maybe even an arris of sap. It is typically silver or grey in colour. If the size of beams required is not a standard square, then it may be cut from standard air dried oak beams and the pieces will have two bright faces and two weathered ones. There may be a few black bore holes, but these are acceptable.
On the other hand, fresh sawn fencing grade European oak beams should not be used for structural purposes. These are of lower grade and are ideal for use in fencing posts.
Fresh sawn joinery grade oak is also suitable for some types of structural uses, and is noted for having fewer knots than structural grade oak. Fresh sawn structural European grade oak beams (also called green oak) are also available as alternatives to air-dried beams. These are newly cut oak beams that may contain some black bore holes without compromising on quality.
Air dried oak beams are the preferred choice of joiners and self-builders looking for high quality oak timber that is durable, stable and with the least number of imperfections. These beams are also easier to work with, making them preferable for creating difficult and non-standard structures or shapes. However, green oak or unseasoned oak is preferred for timber frame construction because they dry after construction and tighten the joints as they shrink to create a sound frame.
© 2013 Juana Aman